Investors initially moved into safe, U.S. government-backed bonds because of worries that Greece might default on its debt, despite assurances of help from other European nations.

Those worries began to ease by midafternoon and investors began to shift money out of ultrasafe assets like Treasurys and back into stocks. That pushed Treasury prices slightly lower and their yields higher.

Meanwhile the results of a $13 billion auction in 30-year bonds Thursday came in on par with the market's expectations and had little effect on trading. The sale wrapped up $82 billion in new Treasury issuance this week.

The yield on 30-year bond that matures in February 2040 rose to 4.76 percent in afternoon trading from 4.75 percent late Wednesday. Its price fell 10/32 to 97 27/32.

Bonds have been in high demand this week, with Wednesday's $21 billion sale of 10-year notes bringing the most bidders. The bid-to-cover ratio on Thursday's sale of 30-year bonds was 2.73. That's in line with recent auctions for bonds with similar maturities.

The yield on the benchmark 10-year note maturing in February 2020 rose to 3.88 percent from 3.87 percent. Its price fell 5/32 at 97 29/32. The yield of the 10-year note is linked to interest rates on mortgages and other consumer loans.

Treasury yields had climbed steadily in recent weeks, in part because of weak demand at some auctions. The 10-year yield rose above 4 percent on Monday for the first time since June. It has not ended a day above 4 percent since October 2008, just before the credit crisis peaked.

In other trading, the yield on the two-year note that matures in March 2012 was unchanged at 1.07 percent. Its price fell less than 1/32 to 99 28/32.

The yield on the three-month T-bill that matures July 8 rose was unchanged at 0.16 percent.

Interest rates rose slightly in the bond market Thursday afternoon, erasing an early morning dip caused by deepening worries about Greece's debt crisis.Investors initially moved into safe, U.S. government-backed bonds because of worries that Greece might default on its...